This invention relates to high voltage protection systems for television receivers.
As is well known, the cathode ray "picture tube" used in a television receiver requires a high direct current accelerating potential between the light-emitting phosphor screen deposited on the picture tube faceplate and the electron gun located in the neck of the tube. This potential is variously referred to as the second anode potential, accelerating potential and ultor voltage. Normally it is developed by rectification of the horizontal (line frequency) retrace pulses generated in the deflection system.
Conventional deflection systems are of the reaction-scan type and include an autotransformer which is energized with line frequency current for supplying a magnetic scansion device, such as a yoke located on the picture tube neck, with appropriate currents for producing a magnetic field for deflecting the electron beam emanating from the gun. A portion of the transformer steps up the voltage, and during the retrace interval at the end of each horizontal scan line, a large amplitude voltage pulse is developed upon collapse of the magnetic field. This high voltage retrace pulse is rectified and supplied to the accelerating electrode and phosphor screen of the picture tube. Some systems develop a retrace pulse of substantially the ultor voltage across the transformer, whereas others use multiplying devices for developing full ultor voltage from a lower valued retrace pulse.
It is also common to produce an intermediate level high voltage for other applications in the receiver. This is commonly referred to as a "boost" voltage and its amplitude bears a direct relationship to the amplitude of ultor voltage produced. The ultor voltage in a typical color television receiver may be between 25 and 30 kilovolts and the boost voltage may range around 800 volts. It is also common to develop an intermediate level focus voltage for some picture tube electron guns which may be on the order of 2 KV. The focus voltage is generally derived from the ultor voltage, for example, by an appropriate resistive bleeder network.
The high voltage amplitude in these systems is a function of the horizontal deflection system. The deflection transformer itself is tuned to a harmonic of the horizontal line frequency. Thus changes in tuning caused by changes in component values often give rise to large amplitude increases in the retrace voltage with concomitant increases in the high voltage. The inner and outer conductive coatings on the funnel of the picture tube, in conjunction with the high dielectric of the glass, function as a filter capacitor for the ultor voltage.
Upward changes in the high voltage pose very serious problems including that of damage to the receiver by insulation breakdown of various components. Radiation is also a function of the high voltage present in the receiver. Recently, the U.S. Department of Health, Education and Welfare established compliance standards for television receivers which dictate the maximum allowable radiation which may be emitted under specified fault conditions as well as other safety standards. Becuase of the possible consequences, should receivers be found in "non-compliance" with these standards, it is prudent for manufacturers to take precautions to assure that the receiver high voltage is controlled and that no receiver will be in non-compliance.
As would be expected, a number of techniques and circuit arrangements have evolved for controlling ultor voltage. Even before the government standards, control of high voltage for the purpose of improving receiver performance and optimizing the reproduced display was an important goal of television manufacturers. (Good high voltage regulation helps maintain picture, brightness, size, stability and focus, for example.) Such control circuits sense a characteristic of the deflection system output, such as the high or boost voltage amplitude, and use it in a control loop to adjust the horizontal drive. While the circuits generally perform well, they ordinarily cannot satisfy the stringent "failure-proof" requirements of the government standards, which demand compliance even under imposed fault conditions. Thus the art has been forced into elaborate "fail-safe" systems of high voltage control.
U.S. Pat. No. 3,789,260, issued 1/29/74, discloses a circuit arrangement for sensing amplitude changes in the voltage developed across the horizontal deflection yoke capacitor, which voltage is related to the high voltage developed for the picture tube. In the event the sensed voltage exceeds a certain maximum the horizontal synchronization of the receiver is disabled to render the picture unviewable.
U.S. Pat. No. 3,795,767, issued 3/5/74, monitors the receiver boost voltage and disables the video circuit whenever the boost voltage exceeds a predetermined level. Disabling the video circuit removes the picture from the screen.
U.S. Pat. No. 3,881,135, issued 4/29/75, also monitors boost voltage and, in response to a predetermined rise therein, disables the entire deflection system, which, of course, also disables the ultor and boost voltages.
These three patents are believed representative of prior art approaches for precluding receiver operation under an excessive high voltage condition. There are, of course, many circuit arrangements which attempt to directly limit the magnitude of the high voltage. A well-known example is that of the spark gap which is designed to flash over at a predetermined voltage level. The known prior art solutions monitor the magnitude of the high voltage or a voltage related thereto.
It will be readily appreciated that, while the purpose of all these circuits is to preclude a non-compliance condition with the government standards from occurring, it is equally important that the failure-circuit not operate sporadically or erroneously. An unfortunate characteristic of most of the amplitude-responsive detection systems is that they are quite prone to spurious operation from noise and other forms of interference. If precautions are taken to minimize their noise responsiveness, their "safety" value is diminished. Also, due to normal variations in device characteristics and production tolerances, it is very difficult to economically make receivers with prior art protection circuits which will assure safe operation without being prone to spurious actuation. Thus there exists a need for a reliable excessive high voltage shutdown system which exhibits greater immunity to spurious operation than systems of the prior art.